THERMAL FLUID PROPERTY AND FLOW SENSING BY DETERMINING A FREQUENCY

摘要

A heater (702) is included in a Wheatstone bridge (704), which includes a voltage dependent resistor (706) in the leg opposite the heater, which is controlled by a time-varying input signal (712) provided by frequency generator (700). A differential amplifier (708) senses any imbalance and provides the necessary power to balance the bridge via a power input signal (710). The resistance and therefore the temperature of the heater is forced to track the resistance of the voltage dependent resistor (706), i.e. the time-varying input signal. Thus the phase lag between the temperature of the heater and the time-varying input signal is forced to be zero. Processor (740) determines the phase shift in the power input signal relative to a reference or calibration function and relates the phase shift to the thermal conductivity.